The advent of the electric piano has required that the movement of the piano keys be detected in a manner that creates an electrical signal for each of the piano keys. Various methods of creating an electrical signal have been used, and one method of detecting the key movement is to use optical sensors to detect a change of position of the piano key. One conventional piano key movement detecting scheme using optical means is disclosed in Sanderson (U.S. Pat. No. 4,768,412) which discloses a portable music recording device that sits on top of a piano keyboard and uses optical means to detect when a key has been depressed. A "piston" is attached to each of the keys, and a "wiper" is attached to the piston. When a key is in its UP position, light can travel from an LED to a phototransistor; when a key is in its DOWN position, the light is blocked by the wiper.
Another conventional electronic keyboard is disclosed in Tamaki (U.S. Pat. No. 4,974,482) which discloses an electronic keyboard in which a movable shutter plate is mounted on the bottom of each of the piano keys. A "guide block" contains a conduit through which light from an LED is directed onto a phototransistor. A slit is formed in the guide block through which the shutter plate can pass, and when the key is in its UP position, the light path through the conduit is open because the shutter plate is not blocking the light path within the slit. When the key is depressed (i.e., DOWN), the shutter plate blocks the light path, thereby changing the output voltage of the phototransistor.
Other conventional electronic keyboard instruments that use shutters or flags to interrupt light beams are disclosed in McLey (U.S. Pat. No. 4,362,934) and Potts (GB 2 178 216 A).
Some of the conventional electronic keyboard instruments use optical components to sense other portions of the piano action besides the keys themselves. For example, Kani (JP 4-243294(A)) discloses a keyboard instrument in which a screen having variations in color shade, hue, and surface inclination is attached on the back check of the action for each of the keys. An LED shines light upon this screen, and the resulting reflection is detected by a phototransistor.
In each of the conventional electronic keyboard instruments that detect the motion of the keys, additional mechanical components are attached to some portion of the key so that a shutter or flag is ultimately positioned to block a light path somewhere along the travel of the key. Furthermore, more than one phototransistor is used to detect the movement of each key in some of the conventional keyboard instruments. These extra mechanical components are potential problem areas that may go out of adjustment, thereby degrading the proper operation of the keyboard instrument.